Paper is sheet material comprising bonded small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Paper is usually made from cellulose fibers although occasionally, synthetic fibers are used. Paper products made from untreated cellulose fibers lose their strength rapidly when they become wet, i.e., they have very little wet strength. The wet strength of paper is defined as the resistance of the paper to rupture or disintegration when it is wetted with water.
Wet strength of ordinary paper is only about 5% of its dry strength. To overcome this disadvantage, various methods of treating paper products have been employed. For example, it is known that wet strength can be increased by parchmentizing paper in sulfuric acid solution or by surface sizing with animal glue and exposing the glue-sized sheet to high temperatures or to a tanning agent to render the protein insoluble in water.
Currently practiced art applying wet strength resins to paper are either of the "permanent" or "temporary" type, i.e., defined by how long the paper retains its wet strength after immersion in water. While wet strength retention is a desirable characteristic in packaging materials, it presents a disposal problem. Paper products having such characteristics are degradable only under undesirably severe conditions. While some resins are known which impart temporary wet strength and thus would be suitable for sanitary or disposable paper uses, they suffer from one or more serious drawbacks. For example, their wet strength is generally of a low magnitude (about one-half of the level achievable for permanent-type resins); they are easily attacked by mold and slime, or they can only be prepared as dilute suspensions.
Urea-formaldehyde and melamine-formaldehyde resins, among others, have been employed to enhance wet strength. These resin types generally suffer from various disadvantages. Absorbency of the paper may, for example, be reduced. Also, since they are utilized in the acid pH range, the deterioration rate of the paper may be increased. Moreover, because these resins contain formaldehyde, serious practical difficulties arise from environmental concerns in the commercial application of such treatments in paper mills. The use of these resins has declined due to concerns for worker safety and health from the presence of formaldehyde in the workplace.
Polyazetidinium chloride-based resins (polyamidoamine-epichlorohydrin reaction products) are established products marketed principally as wet strength resins. Because they are useful in the neutral to alkaline pH range, the use of these-epoxidized resins instead of urea-formaldehyde resins not only results in less corrosion of the papermaking machinery but also produces a paper product with improved softness. Among the resins of this type which have heretofore been used are resins produced by reaction of polyalkylenepolyamines with halohydrins, resins produced by reaction of polyalkylenepolyamines with saturated aliphatic dibasic carboxylic acids to yield a first stage polyamide product with subsequent reaction of this first stage product with a halohydrin, and resins produced by reaction of polyalkylenepolyamines with saturated or unsaturated aliphatic di- or polycarboxylic acids, or aromatic polycarboxylic acids, followed by reaction of the resulting polyamide with a halohydrin.
Polyazetidinium chloride-based wet strength resins are substantive to fibers of hydrated cellulosic material such as aqueous suspensions encountered in paper mills. In addition to being self-retaining on cellulose fiber, such resins undergo homo-cross-linking reactions upon a rise in pH, loss of water, and/or increase in temperature to form an insoluble polyamide network. The curing of this type of resin in paper gives rise to orders of magnitude increases in the paper's tensile strength when wetted with water, with levels of wet strength achieved related to amount of resin applied to the paper. U.S. Pat. No. 2,926,154 discloses this water-soluble, cationic, thermosetting polymer produced as a polymeric reaction product of epichlorohydrin and a polyamide derived from a polyalkylene polyamine and certain dicarboxylic acids.
An unfortunate side effect of imparting wet strength to paper through the use of conventional polyazetidinium chloride-based resins is the resulting difficulty experienced in reclaiming or recycling the paper by repulping the material back to its individual fibers. Achieving de-fibering of wet strength treated paper involves exposing the sheet to sufficient heat and to chemical conditions adequate to initiate or maintain amide hydrolysis, while subjecting the paper to the necessary physical forces to break apart the fiber network without appreciably damaging the fibers themselves. The most common approach is to use an alkali (pH &gt;10), but use of oxidants, such as hypochlorite, persulfate, etc. is sometimes employed with bleached paper. The wet strength imparted to the paper through the use of these resins is of the permanent type.
U.S. Pat. No. 3,372,086 discloses a cationic water-soluble polymer produced in a two-stage reaction system. In the first stage, a polyalkylenepolyamine is reacted with a saturated aliphatic dialdehyde. The resulting resin is then reacted with a halohydrin to give a wet strength resin. More specifically, a polyalkylenepolyamine such as diethylenetriamine is treated with a saturated aliphatic dialdehyde of 2 to 6 carbon atoms such as glyoxal in a proportion of about 0.5 to 2.0 moles of polyalkylenepolyamine per mole of dialdehyde at a temperature of 50.degree.-200.degree. C. for at least 30 minutes and usually for several hours until all the water of reaction has been removed to form a first resin.
This first resin is diluted with water to a solids content of 5-35% and then partially crosslinked by reaction with a halohydrin such as epichlorohydrin, the crosslinking taking place through secondary amine groups of the first resin. The halohydrin is added in an amount sufficient to provide a molar ratio of halohydrin to secondary amine of the first resin of about 0.5 to about 1.5. The time of reaction is 10 minutes to 24 hours, depending on temperature. The resin is then cooled and diluted with water to a solids content of 20% or less and then stabilized by adjusting pH to about 5 with acid. The thus obtained resin is described as a cationic water-soluble thermosetting resin that is cured to a water-insoluble form subsequent to its incorporation in the paper product to effect increased wet strength.
U.S. Pat. No. 3,607,622 discloses a wet strength resin prepared by reacting a polyamidoamine with acrylamide, and then reacting the resulting adduct with a polyaldehyde. Polyamidoamines resulting from the condensation of a polyalkylene polyamide with an organic polybasic acid are suitable. The polyamidoamine is reacted with an amount of acrylamide sufficient to react with substantially all the amine groups in the polyamidoamine. It is disclosed that a lightly alkylated resin can be prepared by adding a small amount of epichlorohydrin during or after reaction of the polyamidoamine with the acrylamide. The polyamidoamine adduct is then reacted with a polyaldehyde.
U.S. Pat. No. 3,914,155 describes a resin for imparting wet strength to paper products. A polyamide is formed by reacting a polyalkylene polyamine with a dicarboxylic acid. The dicarboxylic acids and polyamines are employed in approximately stoichiometric proportions so that essentially all the primary amine groups are converted to amide groups. The ratio of polyamine to dicarboxylic acid is 0.8:1 to 1.4:1. The resulting polyamidoamine is then reacted with formaldehyde at a formaldehyde to secondary amine ratio of 0.5:1 to provide a polyamidol having tertiary amine groups containing --CH.sub.2 OH substituents. The polyamidol is then reacted with epichlorohydrin at an epichlorohydrin to amine ratio of 1:1 to 2.5:1 at 40.degree.-100.degree. C.
U.S. Pat. No. 4,233,411 discloses a wet strength resin prepared by reacting polyacrylamide, glyoxal and a cationic regulator selected from the group consisting of a low molecular weight dimethylamine-epichlorohydrin copolymer, a low molecular weight ethylene dichloride ammonia condensation polymer, and a polyvinyl benzyl trimethyl ammonium chloride polymer.
U.S. Pat. No. 3,556,932 discloses ionic (cationic or anionic) water-soluble resins prepared by reacting a vinylamide polymer with glyoxal. The vinylamide content of the polymer provides sites to which the glyoxal (--CHOHCHO) substituents are attached. The vinylamide polymers have sufficient --CHOHCHO substituents to be thermosetting. Paper prepared using the resin is described as having temporary wet strength and as being especially suitable for re-use when slurried in water having a mildly alkaline pH. The magnitude of the wet strength imparted to the paper is subtle enough in practice that the material is often referred to as a dry strength resin.
U.S. Pat. No. 3,734,977 discloses a water-soluble cationic alkylamine-epichlorohydrin polymer which has been grafted with acrylamide and then reacted with glyoxal. Glyoxalation of the acrylamide units render the polymer thermosetting. The resin is described as retaining wet strength during brief contact with moisture but losing much of its strength after a short period of natural weathering. Again, it is apparent that the reactive aldehyde-based temporary wet strength resins offer ease of reclamation at the expense of wet strength magnitude. In fact, to those of ordinary skill in papermaking, the degree of ease of reclamation is considered inversely related to the level of wet strength present in the paper.
U.S. Pat. No. 4,722,964 discloses a wet strength resin made from a polyalkyleneamine-amide, ammonia and epichlorohydrin. The polyalkyleneamine-amide is made by the condensation polymerization of a polyalkylene polyamine and a diester of a saturated aliphatic dibasic carboxylic acid. The resin is prepared by reacting ammonia with part of the epichlorohydrin to form an intermediate that is then reacted with the polyalkyleneamine-amide and the rest of the epichlorohydrin. Paper treated with the resin is described as being easier to repulp than paper treated with conventional epoxidized polyalkyleneamine-amides.
Increased pressure is being placed on the paper industry by environmentally conscious customers. Consumers are demanding that the paper products they buy be capable of being recycled. In addition, stricter legislative standards are being imposed on the paper industry. For some products containing currently used wet strength resins, recycling is difficult. A need exists in the art for new technology which will address the problems currently associated with the use of conventional polyazetidinium chloride-based wet strength resins.